It has recently become a problem that particulates contained in exhaust gas discharged from internal combustion engines of vehicles such as buses and trucks, construction machines and the like cause harm to the human body and the environment. Various catalyst converters which can collect the particulates in the exhaust gas to purify the exhaust gas have been proposed.
FIG. 10 is a cross-sectional view schematically showing an embodiment of a catalytic converter. In this catalytic converter 10, an introduction pipe 16 through which exhaust gas discharged from an internal combustion engine is introduced is connected to one end of a metal casing 11, and a discharge pipe 17 through which the exhaust gas which has passed through a catalyst carrier 12 is discharged outside is attached to the other end thereof. Further, the catalyst carrier 12 is provided inside the metal casing 11 with the intervention of a holding material 13 for a catalyst converter. Furthermore, an electric heater and a temperature sensor for burning particulates accumulated in the catalyst carrier, namely a honeycomb filter, to recover a filtering function (also referred to as regeneration treatment) may be provided on an exhaust gas introduction side (also referred to as a suction side) with respect to a catalyst carrier, and another pipe for feeding combustion air may be connected thereto, although not shown in the figure. According to such constitution, when the amount of the particulates accumulated in the catalyst carrier increases to result in an increase in pressure drop, the regeneration treatment can be conducted.
Further, the holding material 13 for a catalyst converter is a mat material obtained by forming inorganic fibers to a specified thickness, and the shape thereof is a planar shape, for example, shown in FIG. 11(A). A convex portion 42 is formed on one end of a tabular main body portion 41, and a concave portion 43 having a shape fittable with the shape of the convex portion 42 is formed on the other end. Then, as shown in FIG. 11(B), the main body portion 41 is wound around an outer peripheral surface of the catalyst carrier 12, and the convex portion 42 and the concave portion 43 are engaged with each other, thereby winding the holding material 13 for a catalyst converter around the catalyst carrier 12.
The holding material 13 for a catalyst converter has functions of stably holding the catalyst carrier 12 inside the metal casing 11, maintaining the catalyst carrier 12 at high temperature by heat insulation and keeping a catalytic function good. Accordingly, particularly in a cold region, there is concern that an increase in temperature of the catalyst carrier 12 is inhibited by the influence of outside air temperature, or that the time required for the above-mentioned regeneration treatment increases. Further, the temperature of an outer peripheral portion of the catalyst carrier 12 decreases in regeneration, and unburned particulates remain in this outer peripheral portion to increase a rise in pressure drop. As a result, there is concern that the time taken until the regeneration treatment is required decreases. Furthermore, the temperature difference between the internal temperature (about 600° C.) and the external temperature is large, so that when heat insulation performance of the holding material 13 is not sufficient, thermal stress is generated due to the temperature difference which occurs between a vicinity of a center portion of the catalyst carrier 12 and a vicinity of the outer peripheral portion thereof, resulting in easy occurrence of cracks in the catalyst carrier 12. As a result, there is also concern that durability is impaired.
In recent years, the exhaust gas temperature decreases in many cases, at the time of idling driving in a traffic jam or at the time of engine stop, because some recent automobiles have a function of stopping an engine (idling stop) at the time of stopping, giving consideration to the environment. It becomes therefore more difficult to maintain the catalyst carrier 12 at the treatment temperature even in a region where is not a cold region. Further, it is necessary to early elevate the temperature of the exhaust gas to the treatment temperature of the catalyst carrier 12.
From such a background, countermeasures of defining the thermal conductivity of the holding material 13 for a catalyst converter and the like have been taken (see JP-A-2002-70529 and JP-A-2006-299966).